1. Field of the Invention
The present invention generally relates to surveying instruments and methods of controlling surveying instruments. The invention also relates to a computer-readable carrier embodying a program for controlling surveying instruments. In particular, the present invention relates to surveying instruments which are configured to measure angles and/or distances relative to a target.
2. Brief Description of Related Art
It is known to use surveying instruments for measuring distances between objects and angles in a coordinate system, wherein the surveying instrument is placed at a predetermined position relative to one object and wherein a suitable target is placed at a predetermined position relative to the other object, and wherein the surveying instrument performs an electro-optical measurement, such as determining a distance from the target and determining an angular position of the target within a suitable coordinate system.
From WO 90/12284 there is known a surveying instrument which is configured to perform automatic tracking of a target. The entire contents of this document are incorporated herein by reference. Thus, the surveying instrument may continuously perform measurements with even moving targets. This conventional surveying instrument has a detector which recognizes the target which is to be tracked and controls an actuator control unit such that an orientation of the measuring unit relative to a base is maintained such that the target is always at a center of a field of view of the detector. A problem might arise if a second target, which should not be tracked, appears within the field of view of the detector, where it might happen that the instrument then continues tracking with respect to the second target rather than the first target which should be tracked.